A merino wool T-shirt breathes. Same with linen. Both have "holes" between the fibers, not just in the fabric. It's also warm if it's a bit chilly in a way tha

A merino wool T-shirt breathes. Same with linen. Both have "holes" between the fibers, not just in the fabric. It's also warm if it's a bit chilly in a way that a cotton shirt isn't, again because of the fact that it traps pockets of air between the fibres and that insulates a bit. Thicker wool clothes do the same thing, but more. Because of the "pourousness" and the fact that the wool fibers are covered in natural fats (lanoline) it also dries really fast because the fibers don't suck in the moisture as, say, cotton does. Yes, it won't keep you warm in gale force winds, but I've got a shell jacket for windy days.